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Impedance control of PCB

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  • In blog
  • Monday, 25 June 2018

Impedance control of PCB

       With the increasingly complicated and high speed development of circuit layout, it’s difficult to make sure the signal quality(especially high speed signal). At the same time, impedance control needs to analyse with transmission line theory, it’s the key point to control the impedance matching of signal line, and nerveless impedance control, which is resulting in serious signal reflection and signal distortion, finally it lead to the failure of layout.

        There are some signals requiring for impedance control, such as PCI bus linePCI-E lineUSBEthernetDDR MemoryLVDS signal and so on. Impedance control needs to come true with PCB layout in the finally, having great demands on PCB process by using with EDA software.

       Multilayer_ board’s structure

      It needs to learn with the structure of PCB at first to do well in impedance control for PCB:the multilayer_ board, as they are often called, is made up of core board and prepreg to be laminating, core board is a kind of board with the rigidspecified thicknesscopper-clad in double side, a base material to be the printed circuit board. However, prepreg, to be the wetting layer, playing a role in bonding the core board, the prepreg has some original thickness, but it would make some changes in lamination fabrication.

In general, two dielectric layers in the external of multilayer_ board, using the single copper foil layer as outer copper foil. There are three kinds of specifications of original thickness in the outer and external copper foil, such as 0.5oz1oz2ozand 1oz is about 35um or 1.4mil, the final thickness of outer copper foil is usually adding to 1oz after a series of surface treatment. External copper foil is copper-clad of core in double side, it has little difference between the final thickness and original thickness, but it will reduce some um because of etching.

The outer layer of multilayer_ board is solder mask, as they are often called, is “green oil”, as it has yellow color or other color. The area without copper foil on the surface is slightly thicker than the area with copper foil as it’s difficultly sure that thickness of solder mask, but copper foil is more prominent as it can be felt that when we touch the surface of PCB because of a lack of its thickness.

When manufacturing some thickness of PCB, on the one hand, it needs to choose properly the parameters of various materials, on the other hand, the final thickness of prepreg is smaller than original thickness. There is a typical lamination structure of 6 layers board as the following:

QQ图片20180623092436

Surface copper foil

There are three materials thickness of surface copper foil that can be using, such as 12um18um and 35um. The final thickness of copper foil is about 44um50um and 67um after PCB fabrication.

Core board

The normal base material is s1141A, FR-4, copper-clad in double side, the optional specification is making sure with PCB manufacturer.

Prepreg

The prepreg has four kinds of specifications( original thickness): 7628(0.185mm), 2116(0.105mm), 1080(0.075mm), 3313(0.095mm), the thickness, after pressing to be finishing is usually smaller than the original 10-15um. The maximum of three prepreg could be used in the same wetting layer, and it can’t be the same for its thickness, at least one prepreg is putting into using, but some manufacturers is requiring for two prepreg at least. It can be etching for the copper foil of core board in double side, and bonding with prepreg in double side to be the thicker wetting layer if it’s enough for prepreg’s thickness.

Solder mask

The thickness of solder mask on the copper foil is C210um, C1 where the surface without copper foil is different based on the surface copper thickness, what is that, when the thickness of surface copper is 45um, C113-15um, and when it’s 70um, C117-18um.

Conductor cross-section

The cross-section of conductor is a trapezoidal. Taking top layer for example, the upper bottom edge of trapezoidal is shorten than the lower 1MIL as the the thickness of copper foil is 1OZ. For example, when the line width is 5MIL, the upper bottom edge is about 4MIL, and the lower bottom edge is 5MIL. The bottom edge of upper and lower is different based on the thickness of copper, there is a table as the relationship between the top and bottom of trapezoids in the different situation as the following.

 

 

      Line width

Layer

Base copper thickness(um)

Upper line width(mil)

W2

Lower line width(mil)

W1

Inner layer

18

W-0.5

W

35

W-1

W

70

W-1.5

W-1

External layer

18

W-1

W

35

W-0.8

W-0.5

70

W-1.5

W-1

Wis routing width

 

 

Dielectric constant

The dielectric constant of prepreg is related to its thickness, there is a stable as the thickness of different type prepreg and dielectric constant parameter:

The dielectric constant of base material is related to its resin material, the constant of FR4 base material is 4.2-4.7, and it would be reducing with the increasing of frequency.

Dielectric dissipation factor: dielectric material is usually taking dielectric dissipation factor tanδ to show in the alternating electric field AEF as energy dissipated by heating is called dielectric dissipation. The typical value of S1141A is 0.015.

To make sure that the manufacturing min.line width and spacing: 4mil/4mil.

Impedance calculator’s introduction:

QQ图片20180623092441

It could calculate the impedance with EAD software after learning with the structure of maultilayer_ and master the parameter. Polar SI9000, a good tool to calculate the impedance with Allegro, now there are so many PCB manufacturers using this software.

You would find that there is a little difference between calculating result of Polar SI9000 and Allegro as calculating the impedance of inner signal no matter what differential line and single-ended line, which is related to the treatment in detail, such as the shape of cross-section. However PCBGOGO suggests that you should choose Coated model rather than Surface if calculating the impedance of surface signal, because the Coated model takes solder mask into consideration, it’s more accurate for results. There is a figure shows that the partial screenshots of the surface differential line impedance calculating with Polar SI9000 under the situation of solder mask:

It could take advise of PCB manufacturer because the thickness of solder mask is not easy to control, what’s that, a similar way: subtracting a particular value on the calculating result of Surface model, PCBGOGO suggests that differential impedance is subtracting 8 ohms, and single-ended impedance is subtracting 2 ohms.

 

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